An increasing number of patients who undergo a cardiac CT (computed tomography) examination have metallic implants in their hearts, such as artificial heart valves, bi-ventricular pacemakers, stents, Amplatzer devices or similar. Prospectively gated cardiac CT scanning delivers good image results for soft tissue because the scans are carried out during the cardiac resting phase to so minimize motion-caused image artifacts. Unfortunately, small but non-negligible motion of the metallic implant inside the gating window may still lead to image artifacts. A cardiac computer tomography system is disclosed in Applicants' WO 2007060572.